In a typical wireless, cellular or radio communications network, wireless devices, also known as mobile stations, terminals, and/or User Equipment, UEs, communicate via a Radio-Access Network, RAN, with one or more core networks. The RAN covers a geographical area which is divided into cells, with each cell being served by a base station, e.g. a radio base station, RBS, or network node, which in some networks may also be called, for example, a “NodeB”, “eNodeB” or “eNB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. One radio base station may serve one or more cells.
A Universal Mobile Telecommunications System, UMTS, is a third generation mobile communication system, which evolved from the second generation, 2G, Global System for Mobile Communications, GSM. The UMTS terrestrial radio-access network, UTRAN, is essentially a RAN using wideband code-division multiple access, WCDMA, and/or High-Speed Packet Access, HSPA, to communicate with user equipment. In a forum known as the Third Generation Partnership Project, 3GPP, telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN, as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller, RNC, or a base station controller, BSC, which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System, EPS, have been completed within the 3rd Generation Partnership Project, 3GPP, and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio-Access Network, E-UTRAN, also known as the Long-Term Evolution, LTE, radio access, and the Evolved Packet Core, EPC, also known as System Architecture Evolution, SAE, core network. E-UTRAN/LTE is a variant of a 3GPP radio-access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base station nodes, e.g. eNodeBs in LTE, and the core network. As such, the Radio-Access Network, RAN, of an EPS has an essentially flat architecture comprising radio base station nodes without reporting to RNCs.
Many modern wireless technologies use multiple antenna techniques for efficient transmission. There are several ways to utilize multiple antennas, such as, for example, transmit diversity, beam-forming and spatial multiplexing. However, multiple antennas may also be used for interference mitigation. In this case, when a network node determines the precoders to be used for transmissions to a wireless device, the decision may be based on different strategies. One example of a strategy may be to optimize the signal towards the served wireless device, and another example of a strategy may be to minimize the interference to other wireless devices. One way to reduce the interference for another wireless device is to align the interference with other interfering signals to that wireless device. This is because the several interfering signals may then be cancelled at the wireless device using, for example, an Interference Rejection Combining, IRC, receiver. However, to achieve a total or full interference alignment, coordinated precoder settings between multiple transmitting network nodes is required. This may, for example, be achieved through iterative precoder calculation methods or by connecting the network nodes into clusters.
The gain from using multiple antennas when transmitting from a network node depends on the scenario. For example, in a Line-of-Sight, LoS, situation, i.e. where there is nothing blocking the transmission path between the network node and the wireless device, the channel correlation is typically higher due to reduced angular spread and the channel quality is typically higher. In these conditions, the gain of using spatial multiplexing is often low, which means that beam-forming may preferably be used instead. Here, in a case where two transmitting antennas are used, an achievable gain by using beam-forming is about 3 dB. This means that the average loss by not implementing such a beam-forming precoder and instead using a randomly selected precoder in this case is never more than 3 dB. It should also be noted that the corresponding throughput gain from a 3 dB signal-to-interference-and-noise ratio, SINR, gain is further reduced as SINR levels increase. Practical limitations, such as, e.g. the use of the highest Modulation and Coding Scheme, MCS, further limits the gain achievable at higher channel quality levels.
If multiple antenna sets with orthogonal polarizations are used, the achievable beam-forming gain per polarization is still 3 dB. Furthermore, the inter-polarization coupling is not heavily dependent of the precoding. Therefore, the above reasoning around the effect of a randomly selected precoder may also be applied per polarization in an antenna configuration with more than one polarization.